1. Field of the Invention
The present invention relates to SRAM or a semiconductor device provided with SRAM, especially to SRAM memory cell construction and a method for manufacturing the same.
2. Description of Related Art
Recently, high integration of semiconductor device is going to be advanced, so there is coming same trend to do in the SRAM technology. The most effective influence on the integration of SRAM is reduction area of a memory cell, so that it is thought that manufacturing technology may be the key to realize the high integration.
Therefore, in a SRAM memory cell comprising a pair of cross-coupled invertor, there has been used a three-dimensional construction wherein a load part is laminatedly formed on a balk transistor. As the load part, there are proposed two types, one is a polysilicon high resistor type and the other is a P-ch thin film transistor(TFT) type.
In the latter case, as shown in FIG. 18, which is an equivalent circuit view of a CMOS type SRAM, wherein the load part is a P-ch TFT and the balk part is a N-ch FET. That is, the SRAM memory cell generally comprises four N-ch transistors for the balk part, two of them being driver transistors N1, N2 and the other two access transistors N3, N4, while it comprises two load P-ch transistors P1, P2 for the load part, and further bit lines and word lines for read/write the data and a source for providing voltage Vcc. Each one of the driver transistors N1, N2 and the load transistors P1, P2, for example the transistors P1 and N1 combine to constitute an Invertor circuit (NOT circuit) and a pair of the Invertor circuits combine to constitute Flip-Flop circuit, wherein either one of the two invertor circuits can keep On-state to act as a memory and also when either one of P-ch transistor and N-ch transistor of each invertor circuit is On-state, the other is OFF-state, so that CMOS FET cells use little power and are used in large memories.
However, the conventional two type load ones have the following problems.
In the case of P-ch transistor load type, connection between the N-ch driver transistor and the P-ch load transistor makes PN junctions which have parasitic capacitance generated as shown FIG. 24, resulting in voltage drop which causes to prevent from depressing lower limit hold voltage and lower limit operating voltage (Vcc Low). On the other hand, in the case of polysilicon high resistor type, it is difficult to form a thin layer of polysilicon so that high resistivity of more than several T (10.sup.12).OMEGA. is hardly obtained and the capacitance between nodes becomes smaller due to pattern layout, resulting in lower resistance to soft error.